The object of this invention is to provide a simple means of measuring and thereby adjusting an axis to be normal to a plane, such as the cutter spindle axis and table of a vertical milling machine. Whereas it might be assumed that the spindle of a vertical milling machine would be vertically fixed during its manufacture, this is not necessarily true. There are approximately one million vertical milling machines in the world with cutting spindles that are purposely made adjustable about two mutually orthogonal axes through large angles e.g. 45.degree.. These machines are patterned after and functionally unchanged from a machine first introduced by the Bridgeport Milling Machine Co. in 1938. Although the dominant day to day use of a vertical milling machine occurs with the spindle axis normal to the work table (vertical), there are frequent machining operations in which it is far easier to incline the spindle through an angle than to incline the work piece. When the machining operations that require an inclined spindle are completed, it is then necessary to return the spindle to a vertical position. In the parlance of the machinist, this is called "sweeping or squaring the head". This process of squaring the head is difficult and time consuming for even the best machinist, due to a design characteristic of the two axes about which the spindle of a Bridgeport type machine is adjustable. A better understanding of the problem which said invention remedies can be had by reference to FIG. 1A. In FIG. 1A the work holding milling machine table 10 has defined motion along three mutually orthogonal axes 11, 12, and 13.
The spindle 14 which holds the cutter is mounted above the table. The axis of spindle rotation 15 is normal to the table 10 which gives rise to the name "vertical milling machine". In the Bridgeport type milling machine the spindle can be inclined with respect to the work table and then rigidly secured in an inclined position about two mutually orthogonal axes designated 16 and 17. A significant difference between the two adjustment axes 16 and 17, is that the axis 16 intersects with the spindle axis 15 and adjustment axis 17 is displaced a distance "R" from the spindle axis. In the prior art, "sweeping" or "squaring" the head is done by mounting a dial indicator 18 (FIG. B) at some radius "r" from the spindle axis and then lowering the spindle until the dial indicator makes contact with the work table. When the indicator is in contact with the work table the spindle is then manually rotated, while the operator observes the indicator. If the indicator reads the same at positions 19 and 20, the spindle axis is normal to the table. Conversely, in FIG. 1C for a spindle out-of-square, the indicator at position 19 will read a value equal to the distance P and at 20 it will read a value of -P. Since r is constant and more importantly the spindle axis 15 and 16 intersect, the magnitude of P and -P will be identical but of opposite sign. It is intuitively obvious for an operator to simply adjust the spindle about axis 16 while sweeping the indicator between 19 and 20 until the value of P and -P converge to a zero indicator reading. Practically speaking, adjustment is made until there is no change in indicator reading between 19 and 20. The features of this invention would greatly speed up the process of adjusting a spindle normal to the work table when the spindle axis 15 and the adjustment axis 16 intersect (by elimination of the sweeping process), although this example of spindle construction presents a rather simple problem.
Whereas in the aforementioned case of squaring about the axis 16, "out-of-square" indicator readings will be of equal magnitude but opposite sign; in the case about the axis 17, indicator readings become obscured by a component of spindle translation induced by reason that the spindle axis 15 and adjustment axis 17 are not in the same plane. The axes 15 and 17 are separated by a distance R which is inherent in milling machines of this type. This condition, wherein measurement of spindle perpendicularity is shrouded in indicator values that contain a translational component, is rarely understood by the machinist and invariably results in an exasperating situation for even the most experienced operator. Adjustment about the axis 17 becomes a series of trial adjustments until the operator is either lucky or just yields to a not so square spindle, i.e. good enough.
This invention provides an instrument that eliminates the translational component, thereby allowing adjustment about both axes 16 and 17 with equal simplicity. This invention further provides a unique type of displacement indicator that has been human factored to display both a coarse and fine (low and high sensitivity) displacement readout that is presented to the machinist in a viewing plane coplanar with actual spindle tilt, and further that such indicator has pointer hands that provide a magnified indication of which way the spindle is inclined.